A pumping system is typically used to lift oil and other wellbore fluids from a subterranean reservoir to the surface. One commonly used pumping system is known as a “sucker rod” pump. A sucker rod pumping system incorporates a downhole reciprocating pump comprised of a reciprocating piston inside a pump barrel that is attached to a production tube. The barrel is located in a subterranean reservoir which is at least partially filled with the well bore fluids. The piston is linked to a prime mover at the surface by a mechanical system that translates the rotational movement provided by the prime mover to the reciprocal movement required for the pump piston. The mechanical mechanism includes a rod string, a polished rod, a bridle, a horsehead, a pivotally supported walking beam and a rotating arm. The rod string is connected to the piston and runs inside the production tube through which the wellbore fluids in the subterranean reservoir are lifted to the surface. The rod string is connected to the polished rod at the surface end of the production tube and the polished rod is attached to the bridle which is coupled to the horse head. The horse head is attached to one end of the walking beam and translates its pivotal movement to the reciprocal movement required for the piston. The rotating arm is connected between the other end of the walking beam and the prime mover. The downward stroke starts at the highest point of the horsehead and continues until the horsehead has reached its lowest point. During the down stroke the rod string and piston in the downhole reciprocating pump descend as gravity pulls them downward. The upstroke is powered by the prime mover, which lifts the rod string and piston upward until the horsehead has reached its highest point again.
As the piston descends on the down stroke a check valve (sometimes called the delivery valve or traveling valve) in the piston opens to let wellbore fluids in the barrel pass though. At the same time a check valve (sometimes called the inlet valve or standing valve) in the barrel closes to prevent wellbore fluids in the barrel from escaping into the subterranean reservoir surrounding the barrel. As the piston is raised on the up stroke the delivery valve is closed such that wellbore fluids that are above the piston are lifted upward into the production tube and towards the surface. At the same time the piston is being raised on the up stroke the inlet valve in the barrel opens permitting wellbore fluids in the subterranean reservoir surrounding the barrel to be sucked into the barrel. The cycle described here repeats during each complete stroke of the sucker-rod pumping system.
To operate a sucker-rod pump in a cost effective manner, the pump fillage level and speed of the stroke should be set such that a profitable amount of wellbore fluid can be extracted by the pumping system while avoiding conditions where the well is pumped off. A pump off condition occurs when the rate at which the subterranean reservoir is supplying wellbore fluids to the barrel is exceeded by the rate at which wellbore fluids are being pumped to the surface. When a well is operating in a pumped off condition it is not operating in an effective and efficient manner. If the well is allowed to continue operating in a pump-off condition damage to the rod string and the downhole reciprocating pump will most likely occur. Any damage to the rod string or downhole reciprocating pump will result in down time for the well and expensive repairs to the damaged components. Therefore, an accurate means for determining the wellbore fluid level, pump fillage and adjusting the speed of the pumping system to maintain a cost effective operating level is desirable.